1. Field of the Invention
The present invention relates to a fixing apparatus for applying heat to an image formed on a sheet conveyed to a nip portion of the apparatus, thereby fixing the image onto the sheet.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, a toner image is formed on a recording sheet, and heat and pressure are applied to the toner image by a fixing apparatus, whereby the toner image is fixed onto the sheet.
Conventionally, as such a fixing apparatus, a roller fixing type apparatus and a belt fixing type apparatus have been used (see, Japanese Laid-open Patent Publication No. 11-194647).
In a copying machine having a fixing apparatus of these types, there can occur an abnormality such that a recording sheet is wrapped around a fixing apparatus component at the time of toner melting, irrespective of whether the fixing apparatus is of a roller fixing type or a belt fixing type. Especially, in the case of a thin recording sheet or a recording sheet which is not thin but limp, the wrapped recording sheet becomes inseparably affixed to a roller surface or between belt surfaces due to molten toner.
If such a phenomenon occurs, the wrapped recording sheet is exposed to high temperature environment. It is therefore demanded, from the viewpoint of apparatus safety, that a notification of occurrence of the phenomenon be immediately displayed and the recording sheet in the fixing apparatus be visible so as to be removable from the apparatus without fail.
Heretofore, the fixing apparatus has been configured to have sensors for detecting sheet conveyance timings, which are disposed at the entrance and exit of the apparatus, and timings of a recording sheet being conveyed into and out from the apparatus are monitored by using these sensors. If the recording sheet is not discharged from the fixing apparatus before the lapse of a predetermined time from when the sheet was conveyed into the apparatus, a drive motor performs a braking operation to immediately stop the sheet conveyance (see, for example, Japanese Laid-open Patent Publication No. 2004-354983).
As the drive motor for the fixing apparatus, a brushless DC motor is generally used, which is long in service life and high in speed and torque, and either a so-called short brake method or a so-called reverse brake method is used for the braking operation.
The short brake method causes motor windings of respective phases to be short-circuited to perform a braking operation by a torque generated through electromagnetic induction, while consuming rotational energy through the resistance of the motor windings. On the other hand, the reverse brake method performs a braking operation by controlling winding currents by means of semiconductor devices so that a reverse rotational torque is generated (see, Koji Ogino, How to use brushless DC motor, Ohmsha, Ltd., P103).
A required braking time can be made shorter in the reverse brake method than in the short brake method. With the reverse brake method, however, the power supply voltage increases due to winding currents being regenerated to the power supply, and therefore, circuit components (such as semiconductor switching devices and electrolytic capacitors) must have higher withstand voltages. If the braking time is not properly controlled, a problem is posed, for example, that the braking time becomes excessively long to an extent that the drive motor reversely rotates.
In the belt fixing type fixing apparatus, the motor rotation speed and speed reduction ratio are made large or the size of motor rotor is made large in order to increase the drive motor output.
However, an increased motor rotation speed results in an increase of deceleration time in the short brake method, and an increased rotor size results in an increase of the inertia moment of the rotor and an increase of deceleration time in the reverse brake method.
Therefore, in an image forming apparatus mounted with a belt fixing type fixing apparatus, a braking operation by the low-priced short brake method cannot stop a recording sheet at a position where the sheet is visible for removal when the sheet is wrapped around a fixing apparatus component.
In a belt fixing type fixing apparatus, a change in belt tension causes a change in radius of curvature of a belt at the drive roller, thereby affecting the service life of the belt. Thus, the affect of brake on the belt tension must be reduced as small as possible.